docs: def then_by(self, func):
          """
          Subsequent sorting function in ascending order
          :param func: lambda expression for secondary sort key
          :return: SortedEnumerable instance
          """
          if func is None:
              raise NullArgumentError(u"then by requires a lambda function arg")
          self._key_funcs.append(OrderingDirection(key=func, reverse=False))
          return SortedEnumerable(self._key_funcs, self._data)